Magnetic random access memory (MRAM) is a new memory technology that will likely provide a superior performance over existing semiconductor memories including flash memory and may even replace hard disk drives in certain applications requiring a compact non-volatile memory device. In MRAM bit of data is represented by a magnetic configuration of a small volume of ferromagnetic material. Magnetic state of the ferromagnetic material can be measured during a read-back operation. The MRAM typically includes a two-dimensional array of memory cells wherein each cell comprises one magnetic tunnel junction (MTJ) element that can store at least one bit of data, one selection transistor (T) and intersecting conductor lines (so-called 1T-1MTJ design).
Conventional MTJ element represents a patterned thin film multilayer that includes at least a pinned magnetic layer and a free magnetic layer separated from each other by a thin tunnel barrier layer. The free layer has two stable directions of magnetization that are parallel or anti-parallel to a fixed direction of magnetization in the pinned layer which correspond to two logic states “0” or “1”. Resistance of the MTJ element depends on mutual orientation of the magnetizations in the free and pinned layers and can be effectively measured. A resistance difference between the parallel and anti-parallel states of the magnetizations can exceed 600% at room temperature.
FIG. 1 shows a schematic view of memory cell 10 for storing four logic states according to prior art disclosed in U.S. Pat. No. 5,930,164 (Zhu). The cell 10 includes two MTJ elements 11 and 12 formed on a substrate and connected in series and magnetically separated from each other by a conductive layer 13 made of a non-magnetic material. First MTJ element 11 comprises a first pinned layer 111 and a first free layer 112 made of CoFe and NiFeCo, respectively. Both the layers 111 and 112 are about 50 Å thick. A tunnel barrier layer 113 separates the layers 111 and 112 from each other. The layer 113 is made of Al2O3 and has a thickness of 22-30A. Second MTJ element 12 has a second pinned layer 121 and a second free layer 122 separated from each other by a second barrier layer 123. The second pinned layer 121 and the second free layer 122 have 50 Å and 30 Å in thickness, respectively. The second free layer 122 is thinner than the first free layer 112. This difference provides the free layers 112 and 122 of the MTJ elements 11 and 12 with different hysteresis (or switching) characteristics. The second tunnel barrier layer 123 is made thinner than the first tunnel barrier layer 113. That results in different resistance values of MTJ elements 11 and 12. Thickness of the layer 123 is in a range of 15-22 Å. The pinned layers 111 and 121 are magnetically pinned by anti-ferromagnetic layers (not shown), which are placed adjacent to their external surfaces.
A current source 14 is coupled to the MRAM cell 10 to provide a sense current 15 through the MTJ elements 11 and 12 to a common ground terminal 16. A resistance over cell 10 varies according to the magnetic states of the free layers 112 and 122; thereby a voltage output VOUT over the MRAM cell 10 indicates different values. The output signal VOUT is compared to threshold voltages, which are predetermined from hysteresis characteristics of the cell 10 for identification of recorded data. One of several disadvantages of the cell 10 is a large length-to-width aspect ratio of the MTJ elements 11 and 12 that substantially reduces a storage density of MRAM.
FIG. 2 shows a schematic view of a magnetoresistive element 20 comprising two MTJ elements 11 and 12 according to prior art disclosed in U.S. Pat. No. 6,590,806 (Bhattacharyya). The element 20 distinguishes from the cell 10 shown in FIG. 1 by using a common pinned layer 22 for two MTJ elements 11 and 12. The pinned layer 22 has a structure of a synthetic antiferromagnet (SAF). The SAF pinned layer 22 is composed of two magnetic layers 111 and 121 antiferromagnetically coupled to each other through 0.5-1.0 nm thick layer 221 of Ruthenium (Ru) or Copper (Cu). The SAF structure of the pinned layer 22 allows a reduction of length-to-width aspect ratio. However this reduction is not sufficient for high density MRAM.
Both MRAM elements according to prior art shown in FIG. 1 and FIG. 2 employ field induced switching mechanism of the free layers 112 and 122 that is based on use of two orthogonal magnetic fields. The field induced switching mechanism suffers from a high write current, a large and complicated cell design and causes a serious half-selected cells problem in MRAM array and in the memory cells with two free layers, especially. Besides, the memory elements 10 shown on the FIGS. 1 and 20 shown on the FIG. 2 employ magnetic materials with in-plane magnetization anisotropy that limit their thermal stability and scalability at technology node below 90 nm.
What is needed is a simple design of multi-bit memory cell having a high thermal stability, small cell size, excellent scalability and low switching current; the memory cell that does not suffer from a half-selection problem.